Saturday, June 4, 2011

ECG Interpretation Review #22 (AV Block - What Type?)

The lead II rhythm strip below was obtained from a patient on telemetry. Is there AV block?  If so - what type?
Figure 1 - What type of AV block do you suspect? (Reproduced from ACLS-2013-ePub).
- Note - Enlarge by clicking on Figures - Right-Click to open in a separate window.
INTERPRETATION: The rhythm is irregular. The QRS complex is narrow, indicating a supraventricular mechanism. P waves of similar morphology are present - but they do not all seem to be conducting. This suggests that some form of AV block may be present. The most striking finding on this tracing is the presence of group beating (alternating short-long cycles). Recognition of "group beating" should make one suspect that a type of Wenckebach conduction may be present.  Looking closer:
  • As noted - P waves of similar morphology are seen in this lead II rhythm strip.  However, the P-P interval varies.  This suggests that the underlying rhythm is sinus arrhythmia.
  • Beats are being dropped (ie, one would definitely expect the P waves following beats #2 and #4 to conduct). Complete (3rd degree) AV block is unlikely because the ventricular response is so variable). Thus, this tracing most probably represents a type of 2nd degree AV block (See ECG Blog Review #20). The narrow QRS and the presence of group beating suggest Mobitz I as the prime suspect.
  • The KEY lies with analysis of beats #1, #3, and #5.  On close inspection, it can be seen that QRS morphology of each of these beats differs slightly from that of beats #2, #4, and #6 (the latter 3 beats having a shorter R wave and slightly deeper S wave). The PR interval preceding beat #5 is definitely too short to conduct. This implies that this beat (and probably also the other two beats like it [beats #1 and #3]) are junctional escape beats. Supporting this contention is the fact that the R-R interval of the two escape beats that are seen on this tracing (the R-R interval between beats #2-3 and #4-5) is the same, and corresponds to a junctional escape rate of 38 beats/minute. Subsequent rhythm strips on this patient confirmed our suspicion that the arrhythmia was indeed 2nd degree AV block, Mobitz type I (AV Wenckebach).
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COMMENT: This tracing is an excellent example of how in addition to group beating, recognition of subtle differences in QRS morphology between junctional and sinus conducted beats may also provide a major clue to diagnosis.
  • FINAL POINT: It is important to appreciate that the emergence of escape beats in this tracing is appropriate - and that without them the ventricular response would be even slower.  However, their presence makes definitive diagnosis of the conduction disturbance a difficult task from this tracing alone.
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  - See also ECG Blogs #19, 20, 21 - and Section 20.0 in our ACLS-2013-ePub - 
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